Why are Checkpoints Important to the Health of Cells

Cell cycle checkpoints are control mechanisms that hold the progression of the cell cycle to the next stage in the cell cycle until the conditions are favorable. They ensure proper cell division.  The three most important cell cycle checkpoints are the G₁ checkpoint, the G₂ checkpoint, and the spindle assembly checkpoint. G1 checkpoint checks the presence of sufficient raw materials while G₂ checkpoint checks the integrity of DNA and DNA replication errors, and spindle assembly checkpoint checks the proper attachment of sister chromatids to the microtubules.

Key Areas Covered

1. What are the Checkpoints of the Cell Cycle
     – Definition, Types
2. Why are Checkpoints Important to the Health of Cells
     – G1 Checkpoint, G2 Checkpoint, Spindle Assembly Checkpoint

Key Terms: Cell Cycle, G₁ Checkpoint, G₂ Checkpoint, Interphase, Mitotic Phase, Spindle Assembly Checkpoint

What are the Checkpoints of the Cell Cycle

Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle that can hold the progression of the cell cycle to the next stage until the conditions are favorable. The three most important cell cycle checkpoints are G₁ checkpoint, the G₂ checkpoint, and spindle assembly checkpoint.

1. G₁ checkpoint – G₁ checkpoint occurs at the transition of G₁/S.
2. G₂ checkpoint – G₂ checkpoint occurs at the transition of G₂/M.
3. Spindle assembly checkpoint –Spindle assembly checkpoint occurs at the mitotic phase.

The three main checkpoints of the cell cycle are shown in figure 1.

Figure 1: Cell Cycle Checkpoints

Why are Checkpoints Important to the Health of Cells

The progression of the cell cycle is controlled by three main cell cycle checkpoints; G₁ checkpoint, G₂ checkpoint, and the spindle assembly checkpoint.

G₁ Checkpoint

G₁ checkpoint is the main decision point of the progression of the cell cycle. It is the rate-limiting step of the cell cycle known as the restriction point. The G1 checkpoint occurs at the transition stage of the G₁ stage-cell into the S stage. Protein synthesis and DNA replication occur during the G₁ phase. The transition of G₁ phase cell to the S stage is the beginning of the irreversible stage of cell division. Therefore, unexpected problems such as DNA damages should not pass to the S phase of the cell cycle where replication of DNA takes place.

Hence, the size of the cell, amount of nutrients, growth factors as well as DNA integrity are checked during the G₁ checkpoint, and if the cell does not fulfill requirements to enter the S phase, it enters into the G0 phase in which no cell division occurs.

G₀ phase is a specialized cell stage in which the cell undergoes regular metabolism of a particular tissue. The cells that fulfill the requirements at the end of the G₁ phase may pass to the S phase through the G₁ checkpoint.

G₂ Checkpoint

G₂ checkpoint occurs at the transition stage of G₂ phase-cell into the mitotic phase. DNA replicates during the S phase. Before entering the mitotic phase, the integrity of DNA should be checked. If not, the damaged DNA may pass to the next cell generation. Therefore, the errors in the DNA replication and the DNA damages are checked by the G₂ checkpoint. If a particular cell bears these problems, it will not pass to the mitotic phase. The G₂ checkpoint allows the repair of DNA damages by various DNA repair mechanisms. If the damage is reversible, then the cell enters into the mitotic phase. If not, the cell is subjected to programmed cell death (apoptosis).

Spindle Assembly Checkpoint

Spindle assembly checkpoint is also known as the mitotic checkpoint. It checks the attachment of sister chromatids to the microtubules before entering to the anaphase. During anaphase, the spindle microtubules are contracted to separate the sister chromatids from their centromeres. If the microtubules are not attached to each centromere, sister chromatids may not segregate from each other.

Conclusion

Cell cycle is a series of events that occur during the life cycle of the cell. Interphase, mitotic phase, and cytokinesis are the three stages of cell cycle. The requirements of each stage of the cell cycle are checked during cell cycle checkpoints. The three most important cell cycle checkpoints are G₁ checkpoint, G₂ checkpoint, and spindle assembly checkpoints. The size of the cell, amount of nutrients, growth factors as well as DNA integrity are checked during the G₁ checkpoint. G₂ checkpoint checks the errors in the DNA replication and the DNA damages. Spindle assembly checkpoint checks the attachment of sister chromatids to the microtubules before entering to the anaphase.

Reference:

1. “Cell Cycle Checkpoints.” Khan Academy, Available here.

Image Courtesy:

1.”Figure 10 03 01″ By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia